Important defeats on pinning of 2D pancake vortices in highly anisotropic Bi-2212 superconducting matrix with homovalent Bi/La substitution
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This comprehensive study aims to investigate the change of the flux pinning mechanism, electrical and superconducting properties of the inorganic solid Bi-2212 materials with the La inclusions inserted in the crystal structure by means of magnetotransport measurements exerted in the applied magnetic field range of 0-7 T. Crucial characteristics as regards critical transition temperatures (T-c(onset) and T-c(offset)), room temperature resistivities (rho(300K)), residual resistivities (rho(0)), residual resistivity ratios (RRR), irreversibility fields (mu H-0(irr)), upper critical fields (mu H-0(c2)), thermodynamic critical fields (mu H-0(c)), activation energies (U-0), penetration depths (lambda) and coherence lengths (xi) are evaluated from the experimental magnetoresistivity curves and available theoretical approaches. All the properties obtained above confirm the considerable degradation in the pinning of 2D pancake vortices with the increment of the La individuals randomly distributed in the Bi-2212 superconducting system. Thus, the improved recoupling linelike nature among the consecutively stacked layers leads to decoupling of the adjacent layers and suppression of the interlayer Josephson coupling length as a result of the enhancement in the thermal fluctuations of flattened pancake vortices. The long and short of it is that the La impurities undermine the elasticity of the vortex lattice. For example, the T-c(offset) and T-c(onset) values are found to be about 84.5 K and 85.7 K for the best (pure) sample while the material exposed to the La content level of x = 0.2 obtains the values of 82.2 K and 15.1 K, respectively at zero applied magnetic field. In the case of the applied magnetic field of 7 T, the T-c(offset) value reduces considerably towards the lower temperature values of 72.6 K for the former compound. However, the values belonging to the worst sample (La4) cannot be determined at higher external magnetic field value such as 1 T due to the hybridization of La-3/states with the Cu3d-O2p states in the Cu-O-2 layers (dissipation). This is attributed to the existence of the pair-breaking mechanism in Bi-site La substituted Bi-2212 systems. Moreover, the flux pinning energy values calculated from thermally activated flux flow (TAFF) model decrease dramatically with the enhancement of both the La content level and external magnetic fields. Numerically, the activation energy value is calculated to be about 5173 K (1077 K) for the pure sample at the field value of 0 T (7 T) whereas the values pertaining to the compound substituted by the La concentration level of x = 0.15 are found to be in a range of 1196-281 K. The lack of the experimental evidences does not enable us to find the energy values belonging to the La4 sample at higher applied field. As for the critical field parameters (mu H-0(c1), mu H-0(c), mu H-0(irr), mu H-0(c2) and mu H-0(c3)) and the extracted values (lambda, xi and kappa), the changes in the parameters given also verify the degradation of the flux pinning ability (random distribution of the artificial pinning centers) with the La concentration as a result of the Cooper pair-breaking mechanism. To sum up, the La impurities are unfavorable for the potential applications of these materials in the commercial sector at high temperatures and magnetic fields. (C) 2015 Elsevier B.V. All rights reserved.